The synthesis of heat-shock proteins (hsp) in cells exposed to stress is one of the most highly conserved regulatory systems known and apparently protects cells against the effects of adverse environmental conditions. The process of spermatogenesis is unusually sensitive to slight elevations in temperature and to many toxic agents. However, we have shown that one of the most abundant proteins (P70) in mouse spermatogenic cells is related closely to hsp70, the major inducible hsp. P70 and hsp70 have almost identical mass and isoelectric point. P70 reacts strongly with a monoclonal antibody that is specific for products of the hsp70 gene family. Both P70 and hsp70 are ATP-binding proteins and are purified by using ATP-affinity chromatography. However, P70 and hsp70 are unique proteins on the basis of peptide map analysis and are regulated differently in germ cells. By examining purified preparations of spermatogenic cells, we have shown that preleptotene and leptotene-zygotene spermatocytes contain little P70, while relatively large amounts of P70 are present in pachytene spermatocytes and round spermatids. Labeling studies show that P70 is synthesized primarily in pachytene spermatocytes with little synthesis occurring in other stages of spermatogenesis. The synthesis of hsp70 is not detectable in unstressed cells but is induced in all stages of isolated germ cells following heat stress. These results indicate that P70 is expressed in a stage-specific manner during cell differentiation, whereas hsp70 is only synthesized in germ cells in response to stress. Studies are in progress to examine the molecular biology of hsp in spermatogenic cells and to understand the function of these proteins in the testis.